9 billion or 11 billion? The research behind new population projections

Demographers from the United Nations Population Division and several universities published a paper in Science last week that argues the world population is unlikely to stop growing this century. They calculate that there is an 80% probability that world population, now 7.2 billion, will increase to between 9.6 and 12.3 billion in 2100, with the median at 10.9 billion.

Different projections for future fertility rates in countries such as China and Nigeria are one major reason for the difference in projections between IIASA and the UN. Photo Credit: Evgeni Zotov via Flickr

Next month, we will announce the results of our newest assessment at the launch of a new book entitled: “World Population and Global Human Capital in the 21st Century” (Lutz, Butz and KC, Oxford University Press 2014). Contrary to the UN projections, the IIASA medium (most likely) scenario indicates that world population will increase to 9.2 billion by 2050, peak at 9.4 billion around 2070 and start a slow decline to 9.0 billion by the end of the century.

The new UN paper uses a probabilistic approach to global population projections providing quantitative uncertainty ranges. Such an approach was first developed at IIASA. In a 1997 Nature article, IIASA used probabilistic methods to indicate that the doubling of world population was unlikely. And in a 2001 Nature article, IIASA demographers projected that there was an 85% chance that the world’s population would stop growing this century.

The UN and IIASA population projections use very different approaches for defining the assumptions underlying future fertility and mortality trajectories. The new IIASA projections are based on the substantive input of more than 550 experts worldwide who were invited to evaluate in a peer review manner a set of alternative scientific arguments bearing directly on the future demographic trajectories. This was done through an online survey as well as a series of meetings on five continents. The resulting state of our knowledge and substantive reasoning is documented in over 500 pages in the OUP book.

Alternatively, the UN population projections have recently moved away from their earlier expert-based assumptions to the other extreme: Their new probabilistic population projections reflect expert judgment only in the design of a specific statistical model which then is applied to national time series of 60 years (1950-2010) to extrapolate 90 years (2010-2100) into the future. There is no room for country-specific expert knowledge or for substantive considerations.

IIASA population projections explicitly include education, which is one factor that leads to lower fertility rates and lower projections by IIASA compared to the UN. Source: Wittgenstein Centre Data Explorer

There are two other factors explaining the difference: One is that IIASA now systematically adds a differentiation by level of education in addition to the conventional age and sex to its population projections, as education significantly influences fertility rates (Policy Brief: Rethinking population policies). Once this important source of population heterogeneity is explicitly taken into account the future looks different. In the example of Nigeria, the UN projects an increase from 160 million in 2010 to 914 million in 2100 while IIASA projects only 576 million. The IIASA projections do consider the fact that recently Nigeria has made significant progress in girls education, such that today half of the women aged 20-24 already have secondary education, while among women aged 40-44 the percentage is only 25 percent. And since more educated women consistently have lower fertility, future fertility is likely to decline as the more educated girls enter reproductive age. Disregarding this important structural change leads to higher projections of future fertility.

Another difference lies in the reading of the current fertility levels in Africa as well as in China. The UN assumes that fertility in Nigeria has been stagnant at 6 children per woman for the past decade and for this reason their purely statistical model results in very slow future decline. However, the most recent Demographic and Health Survey (DHS 2013) for Nigeria shows that fertility has already declined to 5.5—a level the UN assumes would only be reached by 2020-25.

The same is true for other African countries such as Mali where the DHS shows fertility has already fallen to 6.1 a value that according to the UN projections would only be reached in 2025-30. For China, currently still the world’s biggest country, the UN assumes that fertility stands at 1.66 and will not decline further but rather increase in the future. Based on expert reasoning the IIASA projections assume that fertility in 2010 was around 1.5 and will decline to 1.4 in the coming decades, following the patterns of Japan, South Korea, Taiwan and Hong Kong which are currently in the 1.0 – 1.4 range.

Population pyramids for Nigeria show IIASA’s projected population and education levels for 2010 and 2050. Source: Wittgenstein Centre Data Explorer